1. Field of the Invention
The invention relates to an audio frequency recovery-DC restorer circuit after demodulation.
2. Discussion of Related Art
For standard narrowband FM, FSK or PSK of cordless phones after demodulation, a conventional AC coupling capacitor method is used to remove the DC content off of the baseband signal (voice and/or data) before passing the signal to the microprocessor or to the AF amplifier. For a more sophisticated high data rate communication, as exemplified by Time Division Duplex (TDD), the following problems arise if such a conventional approach is employed, which consequently reduces quality performance of the cordless phone.
(a) Sensitivity
(b) Distorted demodulated AF signals
(c) Less accurate duty cycle of the replicated digital data.
(d) DC offsets due to frequency offsets between the transmitter and the receiver.
(e) DC offsets due to center frequency inaccuracies in a quadrature resonator.
(f) Time Division Duplex (TDD) burst DC ripple that is induced by transient frequency or DC voltage shifts.
The AC coupled circuit basically skews or distorts the demodulated signal. As a consequence, the signal becomes inefficient for use by the receiver systems that require precise baseband quality output. Due to the nature of a capacitor, a transient DC response arises to any time varying shifts in the input DC level. This leads to unstable bit duty cycles and thus increased bit jitter.
Conventional cordless phones employ 49 MHz narrowband and into use multiple stages of filters (passive or active) and transistor combinations. It would be desirable to avoid using such multiple stages of filters.